A process described in EP-B-0 017 866 which comprises steps a) to g) enables the preparation of anhydrous formic acid starting from methyl formate. Anhydrous formic acid is obtained here if                a) methyl formate is subjected to hydrolysis,        b) methanol and excess methyl formate are distilled off from the resultant hydrolysis mixture,        c) the bottom product from the distillation (b), which comprises formic acid and water, is extracted in a liquid-liquid extraction with an extractant which principally takes up the formic acid,        d) the resultant extract phase, comprising formic acid, extractant and some of the water, is subjected to distillation,        e) the top product obtained in this distillation, which comprises water and some of the formic acid, is fed back into the lower part of the distillation column in step (b),        f) the bottom product from distillation step (d), which predominantly comprises extractant and formic acid, is separated into anhydrous formic acid and the extractant by distillation, and        g) the extractant leaving step (f) is fed back into the process.        
In this process, it is particularly advantageous                h) to carry out distillation steps (b) and (d) in a single column,        i) to introduce the water necessary for the hydrolysis in the form of steam into the lower part of the column provided for carrying out step (b),        k) to employ methyl formate and water in the hydrolysis (a) in a molar ratio of from 1:2 to 1:10, and/or        l) to employ, as extractant, a carboxamide of the general formula I          where the radicals R1 and R2 are alkyl, cycloalkyl, aryl or aralkyl groups, or R1 and R2 jointly, together with the N atom, form a hetero-cyclic 5- or 6-membered ring, and only one of the radicals is an aryl group, and where R3 is hydrogen or a C1-C4-alkyl group.        
Steps (a) to (i) of the above-described process disclosed in EP-B-0 017 866 are explained in greater detail below.
Step (a)
The hydrolysis is usually carried out at a temperature in the range from 80 to 150° C.
Step (b)
The distillation of the hydrolysis mixture can in principle be carried out at any desired pressure, preferably from 0.5 to 2 bar. In general, working under atmospheric pressure is advisable. In this case, the temperature at the bottom of the column is about 110° C. and the temperature at the top of the column is from about 30 to 40° C. The hydrolysis mixture is advantageously added at a temperature in the range from 80 to 150° C., and the methanol is preferably removed in liquid form at a temperature of from 55 to 65° C. Satisfactory separation of the mixture into methyl formate and methanol on the one hand and aqueous formic acid on the other hand is possible even using a distillation column which has 25 theoretical plates (the theoretical number of plates is preferably from 35 to 45). Any design can be used for the column intended for step (b), but a sieve-plate or packed column is particularly recommended.
Step (c)
The liquid-liquid extraction of the formic acid from its aqueous solution by means of an extractant is preferably carried out at atmospheric pressure and a temperature of from 60 to 120° C., in particular from 70 to 90° C., in countercurrent. Depending on the type of extractant, extraction devices having from 1 to 12 theoretical separation stages are generally required. Suitable extraction devices for this purpose are in particular liquid-liquid extraction columns. In most cases, satisfactory results are achieved using from 4 to 6 theoretical separation stages.
The choice of extractant is not limited. Particularly suitable extractants are carboxamides of the general formula I given above. Extractants of this type are, in particular, N-di-n-butylformamide and in addition N-di-n-butylacetamide, N-methyl-N-2-heptylformamide, N-n-butyl-N-2-ethylhexylformamide, N-n-butyl-N-cyclohexylformamide and N-ethylformanilide, and mixtures of these compounds. Further suitable extractants are, inter alia, diisopropyl ether, methyl isobutyl ketone, ethyl acetate, tributyl phosphate and butanediol formate.
Step (d)
The extract phase is separated by distillation in an appropriate distillation device into a liquid phase, which generally comprises predominantly formic acid and extractant, and a vapor phase predominantly comprising water and small amounts of formic acid. This is an extractive distillation. The bottom temperature is preferably from 140 to 180° C. A satisfactory separation effect is generally achieved from 5 theoretical plates.
Step (e)
The formic acid/water mixture is generally recycled in vapor form.
Steps (f) and (g)
The distillation device (usually in the form of a column) for carrying out step (f) is advantageously operated under reduced pressure—from about 50 to 300 mbar and correspondingly low head temperatures—from about 30 to 60° C.
Step (h)
This variant of the process relates to steps (b) and (d). The distillation devices for carrying out steps (b) and (d) are arranged in an overall distillation device. The distillation devices here are generally in the form of columns.
Step (i)
In this step, water required for the hydrolysis is provided in the form of steam.
In the process described above, methyl formate is hydrolyzed in a hydrolysis reactor with a molar excess of water. The hydrolysis reaction is preferably carried out as a pure liquid-phase reaction at temperatures from 80 to 150° C. In order to be able to achieve these temperatures of the reaction mixture, the hydrolysis must be carried out at superatmospheric pressure—at atmospheric pressure, the reaction mixture would have a boiling point below the temperature range indicated above. In the subsequent step, the hydrolysis mixture from the hydrolysis reactor is passed into a distillation column for distillative separation. A lower pressure prevails in the latter than comparatively in the hydrolysis reactor. On introducing the hydrolysis mixture into the distillation column, the hydrolysis mixture is thus decompressed suddenly (abrupt drop in pressure). The consequence is vigorous foaming of the hydrolysis mixture introduced into the distillation column. The foaming has highly adverse effects on the separation performance of the distillation internals of the distillation column, since the foaming is associated with intensive back-mixing of the internal gas and liquid streams in the distillation column, which preferably run in countercurrent. In addition, the fluid-dynamic working range is greatly restricted by the foaming, since the foam causes an unacceptable pressure loss in the column. In order to prevent the foaming, a commercially available antifoam can be added to the distillation column, above the feed point for the hydrolysis mixture. This reduces or greatly restricts the foaming, so that the adverse consequences of foaming described above are excluded. Examples of suitable antifoams which can be employed are silicone oils. The costs of the antifoams have an adverse effect on the economic efficiency of the process. Secondly, the addition of the commercially available antifoams is accompanied by introduction of foreign substances into the process, which generally have an adverse effect on the product quality. Since the degradation products of the antifoams must be eliminated from the process, considerable disposal costs arise in the treatment of the corresponding waste water.